30. Orbitals and Organic Chemistry: Pericyclic Reactions

1. 30. Orbitals and Organic Chemistry: Pericyclic Reactions Based on McMurry’s Organic Chemistry, 6th edition

2. Pericyclic Reactions – What Are? • Involves several simultaneous bond-making breaking process with a cyclic transition state involving delocalized electrons • The combination of steps is called a concerted process where intermediates are skipped

3. 30.1 Molecular Orbitals of Conjugated  Systems • A conjugated diene or polyene has alternating double and single bonds • Bonding MOs are lower in energy than the isolated p atomic orbitals and have the fewest nodes • Antibonding MOs are higher in energy • See Figure 30.1 for a diagram

6. 1,3,5-Hexatriene • Three double bonds and six  MOs • Only bonding orbitals, 1, 2, and 3, are filled in the ground state • On irradiation with ultraviolet light an electron is promoted from 3 to the lowest-energy unfilled orbital (4*) • This is the first (lowest energy) excited state • See the diagram in Figure 30.2

7. 30.2 Molecular Orbitals and Pericyclic Reactions • If the symmetries of both reactant and product orbitals match the reaction is said to be symmetry allowed under the Woodward-HoffmannRules (these relate the electronic configuration of reactants to the type of pericyclic reaction and its stereochemical imperatives) • If the symmetries of reactant and product orbitals do not correlate, the reaction is symmetry-disallowed and there no low energy concerted paths • Fukui’s approach: we need to consider only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the frontier orbitals

8. 30.3 Electrocyclic Reactions • These are pericyclic processes that involves the cyclization of a conjugated polyene • One  bond is broken, the other  bonds change position, a new σ bond is formed, and a cyclic compound results • Gives specific stereoisomeric outcomes related to the stereochemistry and orbitals of the reactants

9. Example: Electrocyclic Interconversions With Octatriene

10. Example: Electrocyclic Interconversions with Dimethylcyclobutene

15. The Signs on the Outermost Lobes Must Match to Interact • The lobes of like sign can be either on the same side or on opposite sides of the molecule. • For a bond to form, the outermost  lobes must rotate so that favorable bonding interaction is achieved

16. Disrotatory Orbital Rotation • If two lobes of like sign are on the same side of the molecule, the two orbitals must rotate in opposite directions—one clockwise, and one counterclockwise • Woodward called this a disrotatory (dis-roh-tate’-or-ee) opening or closure

17. Conrotatory Orbital Rotation • If lobes of like sign are on opposite sides of the molecule: both orbitals must rotate in the same direction, clockwise or counterclockwise • Woodward called this motion conrotatory (con-roh-tate’-or-ee)

20. 30.4 Stereochemistry of Thermal Electrocyclic Reactions • Determined by the symmetry of the polyene HOMO • The ground-state electronic configuration is used to identify the HOMO • (Photochemical reactions go through the excited-state electronic configuration )

21. Ring Closure of Conjugated Trienes • Involves lobes of like sign on the same side of the molecule and disrotatory ring closure

22. Contrast: Electrocyclic Opening to Diene • Conjugated dienes and conjugated trienes react with opposite stereochemistry • Different symmetries of the diene and triene HOMOs • Dienes open and close by a conrotatory path • Trienes open and close by a disrotatory path

28. 30.5 Photochemical Electrocyclic Reactions • Irradiation of a polyene excites one electron from HOMO to LUMO • This causes the old LUMO to become the new HOMO, with changed symmetry • This changes the reaction stereochemistry (symmetries of thermal and photochemical electrocylic reactions are always opposite)

29. Rules for Electrocyclic Reactions

30. 30.6 Cycloaddition Reactions • Two unsaturated molecules add to one another, yielding a cyclic product • The Diels–Alder cycloaddition reaction is a pericyclic process that takes place between a diene (four  electrons) and a dienophile (two  electrons) to yield a cyclohexene product Stereospecific with respect to substituents

31. Rules for Cylcoadditions - Suprafacial Cycloadditions • The terminal  lobes of the two reactants must have the correct symmetry for bonding to occur • Suprafacial cycloadditions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the same face of the other reactant

32. Rules for Cylcoadditions - Antarafacial Cycloadditions • These take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on opposite faces of the other reactant (not possible unless a large ring is formed)

34. 30.7 Stereochemistry of Cycloadditions • HOMO of one reactant combines with LUMO of other • Possible in thermal [4 +2] cycloaddition

35. [2+2] Cylcoadditions • Only the excited-state HOMO of one alkene and the LUMO can combine by a suprafacial pathway in the combination of two alkenes

38. Formation of Four-Membered Rings • Photochemical [2 + 2] cycloaddition reaction occurs smoothly

39. 30.8 Sigmatropic Rearrangements • A s -bonded substituent atom or group migrates across a p electron system from one position to another • A s bond is broken in the reactant, the p bonds move, and a new s bond is formed in the product

42. Sigmatropic Notation • Numbers in brackets refer to the two groups connected by the s bond and designate the positions in those groups to which migration occurs • In a [1,5] sigmatropic rearrangement of a diene migration occurs to position 1 of the H group (the only possibility) and to position 5 of the pentadienyl group • In a [3,3] Claisen rearrangement migration occurs to position 3 of the allyl group and also to position 3 of the vinylic ether

43. Sigmatropic Stereospecificity: Suprafacial and Antarafacial • Migration of a group across the same face of the  system is a suprafacial rearrangement • Migration of a group from one face of the  system to the other face is called an antarafacial rearrangement

46. Stereochemical Rules of Sigmatropic Rearrangements H H

47. 30.9 Some Examples of Sigmatropic Rearrangements • A [1,5] sigmatropic rearrangement involves three electron pairs (two  bonds and one s bond) • Orbital-symmetry rules predict a suprafacial reaction • 5-methylcyclopentadiene rapidly rearranges at room temperature

48. Another Example of a Sigmatropic Rearrangement • Heating 5,5,5-trideuterio-(1,3Z)-pentadiene causes scrambling of deuterium between positions 1 and 5

49. Orbital Picture of a Suprafacial [1,5] H Shift

50. Cope and Claisen Rearrangements are Sigmatropic • Cope rearrangement of 1,5-hexadiene • Claisen rearrangement of an allyl aryl ether